This invention relates generally to resistance welding systems and, more particularly, to a system and method of predicting and adaptively controlling resistance weld quality by monitoring the displacement of a movable electrode with respect to the workpiece or other stationary point.
In a typical resistance welding application, and in particular a resistance spot welding application, metallic workpieces are positioned in an overlapping relationship between a pair of axially opposing electrodes. One electrode is substantially fixed in position and the other electrode is movable toward the fixed electrode. To effect welding, the movable electrode is moved toward the fixed electrode, squeezing or compressing the workpieces therebetween. An electrical current is then passed between the electrodes, and the resistance of the workpiece material is used to melt a localized area in each of the workpieces. When the current is interrupted the molten metal solidifies, thereby fusing the workpieces together. Similarly, in a resistance projection welding application, a single movable electrode moves axially toward a platform or base on which the workpieces are supported. Like spot welding, the workpieces are compressed by the movable electrode and an applied current melts a localized area of each of the workpieces which subsequently fuses together.
However, because these processes generally involve relatively high temperatures, occur over a short period of time and involve very small displacements, the development of process monitoring and control systems and methods in such applications has thus been hindered. It has therefore been difficult to adequately ensure good resultant weld quality in this type of application, especially in high speed automated resistance welding processes. Although there have been numerous attempts at performing this function, none have in practice adequately ensured the consistent production of high quality welds.